1. Field of the Invention
This patent specification relates to hydraulic fracture monitoring and other oilfield applications. More particularly, this patent specification relates to systems and methods for fiber-based evaluation, monitoring and/or control of hydraulic fracturing of subterranean rock formations surrounding boreholes, as well as to other applications where a fiber-based device or tool can be pumped into an otherwise inaccessible space.
2. Background of the Invention
Many hydrocarbon reservoirs worldwide have passed peak production. As about 70% of the hydrocarbon present in a reservoir is not recovered by the initial recovery strategies, many challenges and opportunities exist for so-called brownfields concerning the tail production of the field. In formations with low permeability, producing hydrocarbon is difficult. Thus, stimulating techniques are used to increase the net permeability of a reservoir. One of the techniques consists of using fluid pressure to fracture the formation or extend existing cracks and existing channels from the wellbore to the reservoir thus creating alternative flow paths for the oil or, more commonly, gas to be produced at a higher rate into the wellbore. The geometry of the new flow path determines the efficiency of the process in increasing the productivity of the well.
There is a need for characterization of the new flow path geometry. To date, direct measurement is not possible, and the geometry is generally inferred from fracturing models, or interpretation of pressure measurements. Alternatively, micro-seismic events generated in the vicinity of the new fractures are recorded downhole. Interpretation indicates direction, length and height of the fractures. Still, this “hydraulic fracturing monitoring” or HFM technique is an indirect measurement for which interpretation is hard to verify. In addition, it requires the mobilization of costly wireline borehole seismic assets that are not a very good fit for the economics of the hydraulic fracturing market on land; and a nearby offset well is normally required for monitoring.
Proposals have been made to introduce a fiber optic cable and use light to probe the fracture. For example, see: U.S. Pat. No. 6,978,832, and U.S. Patent Application Publication No. 2005/0012036. However, such techniques can be prone to reliability issues due to poor deployment within fractures. A technique described in U.S. Pat. No. 7,082,993 uses a plurality of active or passive discrete devices such as electronic microsensors, radio-frequency transmitters or acoustic transceivers to transmit their position as they flow with the fracture fluid/slurry inside the created fracture. Active discrete devices can form a network using wireless links to neighboring microsensors. An optical fiber can be deployed through the perforations when the well is cased and perforated or directly into the fracture in an open hole situation, thereby allowing length measurements as well as pressure and temperature measurements. However, such techniques may in general be limited due to signal strength and power constraints on the discrete devices; and their cost is also an open question.